The present invention relates to an apparatus and method for removing surface coverings by blasting with dry ice pellets. In particular, the present invention relates to an apparatus for removing surface coverings using reduced-size dry ice pellets that is especially suitable for use in the production of electronic devices.
The use of dry ice pellets for blast cleaning is well known in the art. Examples of conventional pneumatically powered blast cleaning systems are described in U.S. Pat. No. 4,389,820 and U.S. Pat. No. 5,365,699, which are incorporated herein by reference.
In such systems, pellets of dry ice (solidified CO.sub.2) are drawn into a flowing gas stream (typically compressed air) by the action of a venturi in a blast gun, where they are entrained into the gas stream and propelled out of the gun to impinge against the surface to be cleaned. After the pellets collide with the surface, removing unwanted surface coverings by their impact, they sublimate into gaseous CO.sub.2 and become part of the ambient atmosphere. The only residue from this process is the removed surface covering.
The dry ice pellets typically are produced by machines called pelletizers. Conventional examples of these machines are disclosed in U.S. Pat. No. 4,780,119 and U.S. Pat. No. 5,475,981, which are incorporated herein by reference. In these machines, liquid CO.sub.2 is injected into a cylinder, where it solidifies in the form of snow-like solid CO.sub.2 particles. A piston within the cylinder then compresses the solidified CO.sub.2 and extrudes rod-shaped dry ice through orifices in a die at one end of the cylinder. The dry ice rods are either sliced during extrusion or further broken down to form pellets. The pellets used for typical dry ice blasting vary in size depending on the surface conditions to be treated. Rice-sized pellets (about 3-3.5 mm in diameter) are useful for cleaning paint and other surface coverings that might otherwise be cleaned by sand blasting. The smallest pellets produced by conventional means are about 1-2 mm in diameter.
In the manufacture of semiconductors, printed circuit boards, flat-panel displays, and other products in the electronics industry, extremely thin layers of material often must be removed in precisely defined, narrow strips to achieve the desired thin-film configuration or circuitry. Traditionally, etching by acid solutions or by photo-chemical mechanisms has been used in manufacturing these products. For example, in producing a printed circuit board, a thin conductive film is deposited on a substrate, a patterned deposition of "resist" material is made, and the acid or other etching medium is applied. Because the resist material is resistant to the etching process, only the exposed conductor is removed, leaving the resist-covered conductor to serve as the desired circuit.
The electronics industry also has used sand-blasting techniques to produce the desired circuit patterns for some applications, such as flat panel displays. Because of the small dimensions of the circuits, however, sand having a diameter on the order of one micron (0.001 mm) must be used. Using sand blasting techniques to manufacture such products, however, has several drawbacks. For example, the process results in considerable sand residue. In addition, the special micron-size sand is expensive. Furthermore, static charges created by the interaction of the sand with the resist material or the material being removed can damage the resulting electronic circuit.
Although dry ice pellets might be applicable to electronic circuit manufacture without the inherent drawbacks of sand blasting, it is not possible to use conventional dry ice blasting techniques. The basic problem is that conventional pelletizers cannot be used to produce the small-diameter pellets required to remove such finely defined circuit patterns. Solidified CO.sub.2 cannot readily be extruded through sub-millimeter-sized orifices in a die.
Another application in which conventional sand blasting and CO.sub.2 pellet blasting techniques do not work well is in the cleaning of thin layers of metal, for example, in preparation for surface plating of copper layers. The high kinetic energy of these conventional blasting media unacceptably deforms or damages the thin layers during cleaning. Dry ice pellets of smaller mass are required for this application.
A third application in which conventional techniques do not work well is in the cleaning of flux and solder from printed circuit boards. The primary problem with conventional blasting systems is the static charges that are created from the interaction of the relatively dry cleaning media with the circuit boards. These static charges cause severe damage to the electronic components mounted on the boards.
This invention is intended, therefore, to provide a method and apparatus for producing dry ice pellets having a diameter less than 1000 microns (1 mm). The invention also is intended to provide a method and apparatus for removing small-dimensioned surface areas using such sub-millimeter-sized dry ice pellets. Furthermore, this invention is intended to provide an apparatus and method for controlling pellet size so that a single blasting machine can be used for a variety of applications in which optimum pellet size varies. In addition, this invention is intended to provide an apparatus and method for limiting the static charges created during cleaning with dry ice pellets.
Additional advantages of the present invention will be set forth in part in the description that follows, and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the apparatus and method particularly pointed out in the appended claims.